Light emitting diodes and laser diodes are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (LEDs). Light emitting devices generally comprise a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that comprise the active region of the device.
Typically, an LED includes an n-type substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region. In order to facilitate the application of a voltage to the device, an anode ohmic contact is formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact is formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer).
Because it may be difficult to make highly conductive p-type Group III-nitride materials (such as GaN, AlGaN, InGaN, AlInGaN, and AlInN), lack of current spreading in the p-type layer may be a limiting factor in the performance of LEDs formed from such materials. Accordingly, it may be desirable to form an ohmic contact over as much of the surface area of the exposed p-type layer as possible in order to induce current to pass through as much of the active region of the device as possible. However, providing a large anode contact may be detrimental to device performance in some respects. It is typically desirable to extract as much light as possible out of a light emitting diode. Since the anode ohmic contact generally comprises a metal layer, light generated in the active region of the LED may be partially absorbed in the ohmic contact, reducing the overall luminescent efficiency of the device.
In some devices, it may be desirable to form a reflective metal layer over the exposed p-type layer, so that light that would normally exit the device through the p-type layer is reflected back into the device to be extracted through the substrate. However, highly reflective metals such as aluminum and silver do not form good ohmic contacts to p-type nitride materials. Thus, an ohmic contact is typically provided between the p-type nitride layer and the reflector. Reducing absorption in the ohmic contact remains a concern in such devices.
Accordingly, there is a need for improved ohmic contact structures and methods of forming ohmic contact structures on p-type nitride materials.